An internal combustion engine of either the Otto or Diesel variety has an essentially constant maximum displacement or volume. This requires compromises in design to achieve acceptable power output at full throttle and reasonable thermal efficiency at cruising power. It would be desirable to have an engine with differing displacements for those operating conditions. However, the mechanical constraints of a conventional combustion engine do not permit of that possibility.
It is also desirable to separate the process of generating expandable fluid for an engine and the expansion of such fluid. This permits each of the two processes to be more closely optimized. Compression of fluid can be more nearly isothermal, and expansion which approaches truly isentropic expansion may be provided. It may be desirable to avoid recompression of combustion products which costs power. It would also be desirable to recapture heat in the combustion products for enhancing thermal efficiency of an engine.